Ethanol is rapidly becoming a major hydrogen-rich liquid transport fuel around the world. Worldwide consumption of ethanol in 2005 was an estimated 12.2 billion gallons. The global market for the fuel ethanol industry has also been predicted to grow sharply in future, due to an increased interest in ethanol in Europe, Japan, the USA, and several developing nations.
For example, in the USA, ethanol is used to produce E10, a 10% mixture of ethanol in gasoline. In E10 blends the ethanol component acts as an oxygenating agent, improving the efficiency of combustion and reducing the production of air pollutants. In Brazil, ethanol satisfies approximately 30% of the transport fuel demand, as both an oxygenating agent blended in gasoline, and as a pure fuel in its own right. Also, in Europe, environmental concerns surrounding the consequences of Green House Gas (GHG) emissions have been the stimulus for the European Union (EU) to set member nations a mandated target for the consumption of sustainable transport fuels such as biomass derived ethanol.
The vast majority of fuel ethanol is produced via traditional yeast-based fermentation processes that use crop derived carbohydrates, such as sucrose extracted from sugarcane or starch extracted from grain crops, as the main carbon source. However, the cost of these carbohydrate feed stocks is influenced by their value as human food or animal feed, while the cultivation of starch or sucrose-producing crops for ethanol production is not economically sustainable in all geographies. Therefore, it is of interest to develop technologies to convert lower cost and/or more abundant carbon resources into fuel ethanol.
CO is a major, low cost, energy-rich by-product of the incomplete combustion of organic materials such as coal or oil and oil derived products. For example, the steel industry in Australia is reported to produce and release into the atmosphere over 500,000 tonnes of CO annually. Additionally or alternatively, CO rich gas streams (syngas) can be produced by gasification of carbonaceous materials, such as coal, petroleum and biomass. Carbonaceous materials can be converted into gas products including CO, CO2, H2 and lesser amounts of CH4 by gasification using a variety of methods, including pyrolysis, tar cracking and char gasification. Syngas can also be produced in a steam reformation process, such as the steam reformation of methane or natural gas. Methane can be converted to hydrogen and carbon monoxide and/or carbon dioxide by methane reformation in the presence of a metal catalyst. For example, steam reformation of methane occurs as follows:CH4+H2O→CO+3H2  (1)CO+H2O→CO2+H2  (2)
This process accounts for a substantial portion of the hydrogen produced in the world today. Catalytic processes may be used to convert gases consisting primarily of CO and/or CO and hydrogen (H2) into a variety of fuels and chemicals. Micro-organisms may also be used to convert these gases into fuels and chemicals. These biological processes, although generally slower than chemical reactions, have several advantages over catalytic processes, including higher specificity, higher yields, lower energy costs and greater resistance to poisoning.
The ability of micro-organisms to grow on CO as a sole carbon source was first discovered in 1903. This was later determined to be a property of organisms that use the acetyl coenzyme A (acetyl CoA) biochemical pathway of autotrophic growth (also known as the Woods-Ljungdahl pathway and the carbon monoxide dehydrogenase/acetyl CoA synthase (CODH/ACS) pathway). A large number of anaerobic organisms including carboxydotrophic, photosynthetic, methanogenic and acetogenic organisms have been shown to metabolize CO to various end products, namely CO2, H2, methane, n-butanol, acetate and ethanol. While using CO as the sole carbon source, all such organisms produce at least two of these end products.
Anaerobic bacteria, such as those from the genus Clostridium, have been demonstrated to produce ethanol from CO, CO2 and H2 via the acetyl CoA biochemical pathway. For example, various strains of Clostridium ljungdahlii that produce ethanol from gases are described in WO 00/68407, EP 117309, U.S. Pat. Nos. 5,173,429, 5,593,886, and 6,368,819, WO 98/00558 and WO 02/08438. The bacterium Clostridium autoethanogenum sp is also known to produce ethanol from gases (Abrini et al., Archives of Microbiology 161, pp 345-351 (1994)).
However, ethanol production by micro-organisms by fermentation of gases is typically associated with co-production of acetate and/or acetic acid. As some of the available carbon is typically converted into acetate/acetic acid rather than ethanol, the efficiency of production of ethanol using such fermentation processes may be less than desirable. Also, unless the acetate/acetic acid by-product can be used for some other purpose, it may pose a waste disposal problem. Acetate/acetic acid is converted to methane by micro-organisms and therefore has the potential to contribute to GHG emissions.
WO2007/117157 and WO2008/115080, the disclosure of which are incorporated herein by reference, describe processes that produce alcohols, particularly ethanol, by anaerobic fermentation of gases containing carbon monoxide. Acetate produced as a by-product of the fermentation process described in WO2007/117157 is converted into hydrogen gas and carbon dioxide gas, either or both of which may be used in the anaerobic fermentation process.
The fermentation of gaseous substrates comprising CO, to produce products such as acids and alcohols, typically favours acid production. Alcohol productivity can be enhanced by methods known in the art, such as methods described in WO2007/117157, WO2008/115080, WO2009/022925 and WO2009/064200, which are fully incorporated herein by reference.
U.S. Pat. No. 7,078,201 and WO 02/08438 also describe fermentation processes for producing ethanol by varying conditions (e.g. pH and redox potential) of the liquid nutrient medium in which the fermentation is performed. As disclosed in those publications, similar processes may be used to produce other alcohols, such as butanol.
Microbial fermentation of CO in the presence of H2 can lead to substantially complete carbon transfer into an alcohol. However, in the absence of sufficient H2, some of the CO is converted into alcohol, while a significant portion is converted to CO2 as shown in the following equations:6CO+3H2O→C2H5OH+4CO2 12H2+4CO2→2C2H5OH+6H2O
The production of CO2 represents inefficiency in overall carbon capture and if released, also has the potential to contribute to Green House Gas emissions. Furthermore, carbon dioxide and other carbon containing compounds, such as methane, produced during a gasification process may also be released into the atmosphere if they are not consumed in an integrated fermentation reaction.
It is an object of the present invention to provide system(s) and/or method(s) that overcomes disadvantages known in the art and provides the public with new methods for the optimal production of a variety of useful products.